This invention relates to plasma generation equipment, and is particularly directed to probes for detecting the current, voltage, and phase of radio frequency (RF) electrical power that is being supplied to an RF plasma chamber.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. Because there is typically a severe impedance mismatch between the RF power source and the plasma chamber, an impedance matching network is interposed between the two. There are non-linearities in the plasma chamber, and because of these and because of losses in the line and in the impedance matching network, not all of the output power of the RF generator reaches the plasma chamber. Therefore, it is conventional to employ a probe at the power input to the plasma chamber to detect the voltage and current of the RF wave as it enters the plasma chamber. By accurately measuring the voltage and current as close to the chamber as possible, the user of the plasma process can obtain a better indication of the quality of the plasma. This in turn yields better control of the etching or deposition characteristics for a silicon wafer or other workpiece in the chamber.
At the present time, diode detection probes are employed to detect the amplitude of the current and voltage waveforms. These probes simply employ diode detector circuits to rectify the voltage and current waveforms, and deliver simple DC metering outputs for voltage and for current. These probes have at least two drawbacks in this role. Diode detectors are inherently non-linear at low signal levels, and are notoriously subject to temperature drift. The diode detector circuits also are limited to detecting the signal peaks for the fundamental frequency only, and cannot yield any information about higher or lower frequencies present in the RF power waveform. In addition to this, it is impossible to obtain phase angle information between the current and voltage waveforms, which renders the power measurement less accurate.
One proposal that has been considered to improve the detection of RF power has been to obtain digital samples of the voltage and current outputs of a probe, using flash conversion, and then to process the samples on a high-speed buffer RAM. However, this proposal does have problems with accuracy and precision. At the present time, flash conversion has a low dynamic range, normally being limited to eight bits of resolution. To gain reasonable phase accuracy for plasma customer requirements, it is necessary to reach a precision of at least twelve bits, so that a phase angle precision of better than one degree can obtained at full power. In addition, flash converters require an extremely fast RAM in order to buffer a block of samples before they are processed in a digital signal process (DSP), and fast RAM circuitry is both space-consuming and expensive.
Voltage and current probes that are now in existence are limited in their performance by the fact that they can only monitor the voltage, current, and phase angle at one frequency, and even then such probes have a poor dynamic range. Examining a different frequency requires changing out the hardware, which can be costly and time consuming. This means also that good performance will ensue only if the load is linear, which is never the case with a plasma chamber. Unlike capacitors, inductors, and resistors, plasma chambers impose a highly non-linear load, which causes the sinusoidal waveform of the input power to become distorted. This distortion causes the resulting waveform to be a sum of sinusoids, with the frequency of each additional sinusoid being some integer multiple of the input sinusoidal frequency (i.e., harmonics). Conventional probes can provide voltage, current and coarse phase information, at best, for the fundamental voltage and current waveforms. This severely limits the accuracy of the system, and makes accurate and repeatable control impossible when there is a significant amount of voltage or current appearing in the harmonics.
It is an objective of this invention to provide a reliable and accurate probe, at low cost, for detecting the current and voltage of RF power being applied to a plasma chamber and for accurately finding the phase angle between the applied voltage and applied current.
It is a more specific object of this invention to provide a frequency shifting arrangement that converts the voltage and current to a lower frequency baseband signal to facilitate accurate detection of RF current and voltage of the applied power, as well as phase information.
According to an aspect of the invention, a plasma arrangement has an RF power generator that supplies an RF electrical wave at a predetermined frequency to a power input of a plasma chamber within which the RF electrical wave produces a plasma. A plasma probe picks up both an RF voltage waveform and an RF current waveform of the electrical wave. The plasma probe sends the RF voltage and current waveforms to an analysis board which converts the RF waveforms to baseband voltage and current signals. A controllable local oscillator provides a local oscillator signal which is a square wave. A voltage signal mixer has inputs that receive the RF voltage waveform and the local oscillator signal, respectively, and an output that provides an audio frequency (AF) baseband voltage signal. A current signal mixer has inputs that receive the RF current waveform and the local oscillator signal, respectively, and has an output that provides a baseband AF current signal. A stereo A/D converter has a first channel input to which the baseband voltage signal is applied, a second channel input to which the baseband current signal is applied, and a serial output that provides a time-synchronous serial digital signal containing alternate digital representations of the baseband voltage waveform and the baseband current waveform. A digital signal process has an input coupled to the serial output of the stereo A/D converter. The digital signal processor is suitably programmed to take the input AF voltage and current signals, determine the amplitude and relative phase of the voltage and current signals, and compute relative RF parameters based on these signals. An external interface provides an output determination based on the amplitudes and relative phase. A local oscillator interface circuit couples the digital signal processor to the local oscillator so that the digital signal processor can control the frequency of the local oscillator signal. In a preferred embodiment, the local oscillator provides said local oscillator frequency within about 15 KHz of the plasma RF frequency, so that the difference frequency, that is, the baseband frequency of the baseband current and voltage signals, is approximately 0.2 KHz to 15 KHz. Also, the stereo A/D converter is preferably a high-fidelity audio-frequency stereo converter, and can be of the type that is frequently used in high-fidelity audio systems, such as a matched two-channel 20-bit A/D converter. The A/D converters preferably incorporate anti-aliasing filters band-limiting the input baseband signals to the range of 0.2 KHz to 20 KHz. The local oscillator preferably includes a programmable oscillator, and can also include a divide-by-two frequency divider following the programmable oscillator to maintain a constant duty cycle. Information about harmonics can be derived by changing the local oscillator signal to a multiple of the RF waveform frequency plus or minus up to 20 KHz.
According to another aspect of this invention, amplitude and relative phase information for current and voltage can be derived for an RF power wave that is applied at a predetermined frequency to a power input of a plasma chamber within which the RF power wave produces a plasma. A plasma probe picks up an RF voltage waveform and an RF current waveform of the applied power. The technique of this invention involves generating a local oscillator signal and mixing the local oscillator signal and the RF voltage and current waveforms to produce the voltage baseband signal at an audio frequency and the current baseband signal at an audio frequency. A feedback signal is supplied from a digital signal processor to control the frequency of said local oscillator signal. The voltage baseband signal and said current baseband signal are converted to a time-synchronous serial digital signal that is supplied to the digital signal processor, which is suitably programmed to compute the amplitudes and relative phase of the voltage and current baseband signals. The local oscillator signal is produced at a frequency that is within 0.20 KHz to 20 KHz of the predetermined frequency of said RF power wave, so that the baseband signals will have a frequency in the audio range of 200 Hz to 20 KHz. Preferably this is about 10 KHz.
The digital signal processor computes the amplitudes and relative phase of the voltage and current baseband signals, preferably by means of a Fast Fourier Transform (FFT) of the current and voltage baseband waveforms. Then, phase and magnitude measurements of the voltage and current baseband signals are made by tracking the baseband frequency of the current and voltage baseband signals. The phase and magnitude measurements can be carried out after the Fast Fourier Transform by extracting frequency spectra of the voltage and current waveforms from the fast Fourier transform. The extracted spectra are employed to compute the phase difference or phase angle between the voltage and current waveforms.
Computing the amplitudes and relative phase of the voltage and current baseband signals is carried out in the digital signal processor. A predetermined number of samples of the serial digital signal representing the baseband voltage waveform and the baseband current waveform, respectively, are transferred into the DSP, and these samples are multiplied by a predetermined window function to produce windowed current and voltage signals. Then, windowed voltage samples V and associated current samples I are processed as a complex waveform, W=V+j *I (where j is the root of minus one), and the digital signal processor performs a complex Fast Fourier Transform operation FFT(W) on the complex waveform. This produces a complex output, from which the digital signal processor can extract current and voltage spectra. The amplitudes and relative phase of said voltage and current baseband signals can be obtained from vector summation of the voltage and current spectra, and from the arctangent of the resulting vector sums.
From these data, other useful values can be calculated which can be used in the accurate control of the RF plasma process, including but not limited to RMS voltage, RMS current, delivered (dissipated) power, forward power, reverse or reflected power, reactive power, apparent power, magnitude of load impedance, phase of load impedance, load resistance, load reactance, magnitude of reflection coefficient, phase of reflectance coefficient, and voltage standing wave ratio (VSWR).
The above and other objects, features, and advantages of this invention will become apparent from the ensuing description of a preferred embodiment, which should be read in conjunction with the accompanying Drawing.